Microbial catalyzed electrochemical systems: a bio-factory with multi-facet applications

Precious Metal Recovery from Wastewater Using Bio-Based Techniques

The recovery of metals from waste material has been on the increase in the past few years due to a number of reasons such as supporting the diversification of metal supply resources. In addition, the alternative use of the waste material for metal recovery can add to the main production line, boosting production throughput and profitability thus, allowing companies to sustain their activities during times of low commodity prices. While there has been a lot of research and interest in the recovery of precious metals such as platinum group metals (PGMs), Au, and Ag from solid waste material, there has been limited focus on the recovery of these value metals from wastewater. This is mostly related to challenges associated with finding cost-effective technologies that can recover these metals from solutions of low metal concentrations. In recent years, bio-based technologies have, however, become established as potential alternatives to traditional techniques in the treatment of wastewater due to their ability to recover metals from solutions of low concentrations. While wastewater might be characterized by some significant value metal content, it also contains other components that have potential economic value if recovered or converted to by-products. Such an approach may not only provide an opportunity for extraction of metal resources from wastewater but also contributes toward the circular economy. This chapter presents insights into precious metal recovery from wastewater using bio-based technologies, compares such an approach to the traditional techniques, explores the recovery of other value-added products and finally considers some of the challenges associated with the large-scale application of the bio-based technologies.

Emerging trends and advances in valorization of lignocellulosic biomass to biofuels

Sustainable technologies pave the way to address future energy demand by converting lignocellulosic biomass into fuels, carbon-neutral materials, and chemicals which might replace fossil fuels. Thermochemical and biochemical technologies are conventional methods that convert biomass into value-added products. To enhance biofuel production, the existing technologies should be upgraded using advanced processes. In this regard, the present review explores the advanced technologies of thermochemical processes such as plasma technology, hydrothermal treatment, microwave-based processing, microbial-catalyzed electrochemical systems, etc. Advanced biochemical technologies such as synthetic metabolic engineering and genomic engineering have led to the development of an effective strategy to produce biofuels. The microwave-plasma-based technique increases the biofuel conversion efficiency by 97% and the genetic engineering strains increase the sugar production by 40%, inferring that the advanced technologies enhances the efficiency. So understanding these processes leads to low-carbon technologies which can solve the global issues on energy security, the greenhouse gases emission, and global warming.

Electrogenic engineered flow through tri-phasic wetland system for azo dye treatment: Microbial dynamics and functional metagenomics

Electrogenic engineered flow through tri-phasic wetland (EEFW) system based on nature-based ecological principles was studied by integrating successive biological microenvironments. The potential mechanism of the plant root-based microbial community and its functional diversity with the influence of plant-microbe-electrode synergism towards dye degradation was evaluated. The EEFW system was operated at three varied dye loads of 10, 25 and 50 mg L-1, where the results from the cumulative outlets revealed a maximum dye removal efficiency of 96%, 96.5% and 93%, respectively. Microbial community analysis depicted synergistic dependence on the plant-microbe-electrode interactions, influencing their functional diversity and metabolism towards detoxification of pollutants. The core microbial taxa enriched against the microenvironment variation were mostly associated with carbon and dye removal viz., Desulfomonile tiedjei and Rhodopseudomonas palustris in Tank 1 and Chloroflexi bacterium and Steroidobacter denitrificans in Tank 2. The degradation of polycyclic aromatic hydrocarbons, chloroalkane/chloroalkene, nitrotoluene, bisphenol, caprolactam and 1,1,1-trichloro-2,2-bis(4-chlorophenyl) ethane (DDT) were observed to be predominant in Tank 1. EEFW system could be one of the option for utilizing nature-based processes for the treatment of wastewater by self-induced bioelectrogenesis to augment process efficiency.

Microbial Biofilms for Environmental Bioremediation of Heavy Metals: a Review

Heavy metal pollution caused due to various industrial and mining activities poses a serious threat to all forms of life in the environment because of the persistence and toxicity of metal ions. Microbial-mediated bioremediation including microbial biofilms has received significant attention as a sustainable tool for heavy metal removal as it is considered safe, effective, and feasible. The biofilm matrix is dynamic, having microbial cells as major components with constantly changing and evolving microenvironments. This review summarizes the bioremediation potential of bacterial biofilms for different metal ions. The composition and mechanism of biofilm formation along with interspecies communication among biofilm-forming bacteria have been discussed. The interaction of biofilm-associated microbes with heavy metals takes place through a variety of mechanisms. These include biosorption and bioaccumulation in which the microbes interact with the metal ions leading to their conversion from a highly toxic form to a less toxic form. Such interactions are facilitated via the negative charge of the extracellular polymeric substances on the surface of the biofilm with the positive charge of the metal ions and the high cell densities and high concentrations of cell-cell signaling molecules within the biofilm matrix. Furthermore, the impact of the anodic and cathodic redox potentials in a bioelectrochemical system (BES) for the reduction, removal, and recovery of numerous heavy metal species provides an interesting insight into the bacterial biofilm-mediated bioelectroremediation process. The review concludes that biofilm-linked bioremediation is a viable option for the mitigation of heavy metal pollution in water and ecosystem recovery.

Carbon credit reduction: A techno-economic analysis of "drop-in" fuel production

The current study elucidates the fundamentals of technical, financial, and environmental viability of the processes used for sustainable "drop-in" fuel generation. At present, the price of producing "drop-in" fuels is around two times as costly (5-6 USD/gallon) as the cost of fossil fuels (3 USD/gallon), especially when using second-generation feedstocks. Hence, this necessitates a comprehensive techno-economic understanding of the current technologies with respect to "drop-in"-fuel. This entitles technical-economic viability, and environmental sustainability to make the processes involved commercially viable. In this context, the present review addresses unique contrasts among the various processes involved in "drop-in" fuel production. Furthermore, principles and process flow of techno-economic analysis as well as environmental implications in terms of reduced carbon footprint and carbon credit are elucidated to discuss fundamentals of techno-economic analysis in terms of capital and operational expenditure, revenue, simulation, cash flow analysis, mass and energy balances with respect to evidence-based practices. Case specific techno-economic studies with current developments in this field of research with emphasis on software tools viz., Aspen Plus, Aspen HYSIS, Aspen Plus Economic Analyser (APEC) Aspen Icarus Process Evaluator (AIPE) are also highlighted. The study also emphasis on the carbon foot print of biofuels and its carbon credits (Carbon Offset Credits (COCs) and Carbon Reduction Credits (CRCs)) by leveraging a deep technical and robust business-oriented insights about the techno-economic analysis (TEA) exclusively for the biofuel production.

Prospects and trends in bioelectrochemical systems: Transitioning from CO2 towards a low-carbon circular bioeconomy

Resource scarcity and climate change are the most quested topics in view of environmental sustainability. CO2 sequestration through bioelectrochemical systems is an attractive option for fostering bioeconomy development upon several value-added products generation. This review details the state-of-the-art of bioelectrochemical systems for resource recovery from CO2 along with various biocatalysts capable of utilizing CO2. Two bioprocesses (photo-electrosynthesis and chemolithoelectrosynthesis) were discussed projecting their potential for biobased economy development from CO2. Significance of adopting circular strategies for efficient resource recycling, intensifying product value, integrations/interlinking of multiple process chains for the development of circular bioeconomy were discussed. Existing constrains as well as outlook for near establishment of circular bioeconomy from CO2 is presented by weighing fore-sighted plans with current actions. Need for developing CO2-based circular bioeconomy via innovative business models by analyzing social, technical, environmental and product related aspects are also discussed providing a roadmap of gaps to pursue for attaining practicality.

Microbial electrolysis cells for the production of biohydrogen in dark fermentation - A review

To assess biohydrogen for future green energy, this review revisited dark fermentation and microbial electrolysis cells (MECs). Hydrogen evolution rate in mesophilic dark fermentation is as high as 192 m³ H₂/m³-d, however hydrogen yield is limited. MECs are ideal for improving hydrogen yield from carboxylate accumulated from dark fermentation, whereas hydrogen production rate is too slow in MECs. Hence, improving anode kinetic is very important for realizing MEC biohydrogen. Intracellular electron transfer (IET) and extracellular electron transfer (EET) can limit current density in MECs, which is proportional to hydrogen evolution rate. EET does not limit current density once electrically conductive biofilms are formed on anodes, potentially producing 300 A/m². Hence, IET kinetics mainly govern current density in MECs. Among parameters associated with IET kinetic, population of anode-respiring bacteria in anode biofilms, biofilm density of active microorganisms, biofilm thickness, and alkalinity are critical for current density.

Bioelectrochemical systems in aid of sustainable biorefineries for the production of value-added products and resource recovery from wastewater: A critical review and future perspectives

Bioelectrochemical systems (BES) have the potential to be used in a variety of applications such as waste biorefinery, pollutants removal, CO₂ capture, and the electrosynthesis of clean and renewable biofuels or byproducts, among others. In contrast, many technical challenges need to be addressed before BES can be scaled up and put into real-world applications. Utilizing BES, this review article presents a state-of-the-art overall view of crucial concepts and the most recent innovative results and achievements acquired from the BES system. Special attention is placed on a hybrid approach for product recovery and wastewater treatment. There is also a comprehensive overview of waste biorefinery designs that are included. In conclusion, the significant obstacles and technical concerns found throughout the BES studies are discussed, and suggestions and future requirements for the virtual usage of the BES concept in actual waste treatment are outlined.

Role of carbon-dioxide sequestering bacteria for clean air environment and prospective production of biomaterials: a sustainable approach

The increase in demand of fossil fuel uses for developmental activity and manufacturing of goods have resulted a huge emission of global warming gases (GWGs) in the atmosphere. Among all GWGs, CO₂ is the major contributor that inevitably causes global warming and climate change. Mitigation strategies like biological CO₂ capture through sequestration and their storage into biological organic form are used to minimize the concentration of atmospheric CO₂ with the goal to control climate change. Since increasing atmospheric CO₂ level supports microbial growth and productivity thus microbial-based CO₂ sequestration has remarkable advantages as compared to plant-based sequestration. This review focuses on CO₂ sequestration mechanism in bacteria through different carbon fixation pathways, involved enzymes, their role in calcite, and other environmentally friendly biomaterials such as biofuel, bioplastic, and biosurfactant.

Bioelectrochemical systems-based metal recovery: Resource, conservation and recycling of metallic industrial effluents

Metals represent a large proportion of industrial effluents, which due to their high hazardous nature and toxicity are responsible to create environmental pollution that can pose significant threat to the global flora and fauna. Strict ecological rules compromise sustainable recovery of metals from industrial effluents by replacing unsustainable and energy-consuming physical and chemical techniques. Innovative technologies based on the bioelectrochemical systems (BES) are a rapidly developing research field with proven encouraging outcomes for many industrial commodities, considering the worthy options for recovering metals from industrial effluents. BES technology platform has redox capabilities with small energy-intensive processes. The positive stigma of BES in metals recovery is addressed in this review by demonstrating the significance of BES over the current physical and chemical techniques. The mechanisms of action of BES towards metal recovery have been postulated with the schematic representation. Operational limitations in BES-based metal recovery such as biocathode and metal toxicity are deeply discussed based on the available literature results. Eventually, a progressive inspection towards a BES-based metal recovery platform with possibilities of integration with other modern technologies is foreseen to meet the real-time challenges of viable industrial commercialization.

Advances in Metagenomics and Its Application in Environmental Microorganisms

Metagenomics is a new approach to study microorganisms obtained from a specific environment by functional gene screening or sequencing analysis. Metagenomics studies focus on microbial diversity, community constitute, genetic and evolutionary relationships, functional activities, and interactions and relationships with the environment. Sequencing technologies have evolved from shotgun sequencing to high-throughput, next-generation sequencing (NGS), and third-generation sequencing (TGS). NGS and TGS have shown the advantage of rapid detection of pathogenic microorganisms. With the help of new algorithms, we can better perform the taxonomic profiling and gene prediction of microbial species. Functional metagenomics is helpful to screen new bioactive substances and new functional genes from microorganisms and microbial metabolites. In this article, basic steps, classification, and applications of metagenomics are reviewed.

Interrelation between sulphur and conductive materials and its impact on ammonium and organic pollutants removal in electroactive wetlands

This investigation is the first of its kind to evaluate the interrelation of sulphate (SO₄^2-) with conductive materials as well as their individual and synergetic effects on the removal of ammonium and organic pollutants in electroactive wetlands, also known as constructed wetland (CW) - microbial fuel cell (MFC). The role of MFC components in CW was investigated to treat the sulphate containing wastewater under a long-term operation without any toxicity build-up in the system. A comparative study was also performed between CW-MFC and CW, where sulphate containing wastewater (S-replete) and without sulphate wastewater (S-deplete) was assessed. The S-replete showed high NH₄⁺ removal than the S-deplete, and the requesnce of removal was: CW-MFC-replete>CW-MFC-deplete>CW-replete>CW-deplete. The chemical oxygen demand (COD) removal was high in the case of CW-MFC-replete, and the sequence of removal was CW-MFC-replete>CW-MFC-deplete>CW-deplete>CW-replete. X-ray photon spectroscopic study indicates 0.84% sulphur accumulation in CW-MFC-replete and 2.49% in CW-replete, indicating high sulphur precipitation in CW without the MFC component. The high relative abundance of class Deltaproteobacteria (7.3%) in CW-MFC-replete along with increased microbial diversity (Shannon index=3.5) rationalise the symbiosis of sulphate reducing/oxidising microbes and its impact on the treatment performance and electrochemical activity.

Evaluation of cell wall-associated direct extracellular electron transfer in thermophilic Geobacillus sp

In this study, a cell wall-associated extracellular electron transfer (EET) was determined in the thermophilic Geobacillus sp. to utilize iron as a terminal electron acceptor. The direct extracellular transfer of its electrons was primarily linked to the cell wall cytochrome-c and diffusible redox mediators like flavins during the anoxic condition. Based on the azo dye decolouration and protein film voltammetry, it was revealed that, in the absence of surface polysaccharide and diffusible mediators, the cell wall-associated EET pathway was likely to be a favorable mechanism in Geobacillus sp. Since the permeability of such redox molecule is primarily limited to the cell wall, the electron transfer occurs by direct contact with cell wall-associated cytochrome and final electron acceptor. Furthermore, transfer of electrons with the help of redox shuttling molecules like riboflavin from cytochrome to cells, vice versa indicates that Geoabcillus sp. has adopted this unique pathway during an anoxic environment for its respiration.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02917-2.

Emerging trends in microbial fuel cell diversification-Critical analysis

Global need for transformation from fossil-based to bio-based economy is constantly emerging for the production of low-carbon/renewable energy/products. Microbial fuel cell (MFC) catalysed by bio-electrochemical process gained significant attention initially for its unique potential to generate energy. Diversification of MFC is an emerging trend in the context of prioritising/enhancing product output while exploring the mechanism specificity of individual processes. Bioelectrochemical treatment system (BET), microbial electrosynthesis system (MES), bioelectrochemical system (BES), electro-fermentation (EF), microbial desalination cell (MDC), microbial electrolysis cell (MEC) and electro-methanogenesis (EM) are the diversified MFC systems that are being researched actively. Owing to its broad diversification, MFC domain is increasing its potential credibility as a platform technology. Microbial catalyzed electrochemical reactions are the key which directly/indirectly are proportionally linked to electrometabolic activity of microorganisms towards final anticipated output. This review intends to holistically document the mechanisms, applications and current trends of MFC diversifications towards multi-faced applications.

Blending of microbial inocula: An effective strategy for performance enhancement of clayware Biophotovoltaics microbial fuel cells

Performance of clayware Biophotovoltaics (BPVs) with three variants of inocula namely anoxygenic photosynthetic bacteria (APB) rich Effective microbes (EM), Up-flow anaerobic sludge blanket reactor (UASB) sludge, SUPER-MIX the blend of EM and UASB inoculum were evaluated on the basis of electrical output and pollutant removal. SUPER-MIX inocula with microbial community comprising of 28.42% APB and 71.58% of other microbes resulted in peak power density of 275 mW/m², 69.3 ± 1.74% Coulombic efficiency and 91 ± 3.96% organic matter removal. The higher performance of the SUPER-MIX than EM and UASB inocula was due to the syntrophic associations of the various APBs and other heterogenous microorganisms in perfect blend which improved biocatalytic electron transfer, electro-kinetic activities with higher redox current and bio-capacitance. The promising performance of clayware BPVs with SUPER-MIX inocula indicate the possibility of BPVs to move towards the scale-up process to minimize the investment towards pure culture by effective blending strategies of inocula.

Bio-membrane based integrated systems for nitrogen recovery in wastewater treatment: Current applications and future perspectives

Nitrogen removal is crucial in wastewater treatment process as excessive nitrogen content could result in eutrophication and degradation of aquatic ecosystems. Moreover, to satisfy the fast-growing need of fertilizers due to an increase in human population, recovering nitrogen from wastewater is of the most sustainable approach. Currently, the membrane technique integrated with biological processes namely bio-membrane based integrated system (BMIS) is a promising technology for recovering nitrogen from wastewater, including osmotic membrane bioreactors, bioelectrochemical systems and membrane photobioreactors. In this review study, the nitrogen recovery in different BMHSs, the role of operational parameters and the nitrogen recovery mechanism were discussed. Apart from this, the implementation of nitrogen recovery at pilot- and full-scale was summarized. Perspectives on the major challenges and recommendations of the BMIS for the nitrogen recovery in wastewater treatment were proposed, in which the integrated technologies and more scale-up studies regarding nitrogen recovery by the BMISs were also highlighted and recommended.

Bioelectrosynthesis technology for enhancing methane production using a thermophilic methanogenic consortium

This work investigated the electrocatalytic activity of a thermophilic methanogenic consortia (TMC) for developing a bioelectrosynthesis process to convert food and paper wastes to methane. Electroanalytical techniques were used to analyze the electrocatalytic activity of the TMC biofilm formed onto the electrodes. The developed electromethanogenesis process enhanced the yield of methane by 54.7% than control experiments. Scanning electron micrographs of the TMC bioelectrodes showed that the electrosynthesis process accelerates biofilm formation onto the electrodes leading to enhanced direct electron transfer reactions at electrode-electrolyte interface. This study will help in developing a novel approach for valorization of food and paper waste to biofuels.

A Comprehensive Understanding of Electro-Fermentation

Electro-fermentation (EF) is an upcoming technology that can control the metabolism of exoelectrogenic bacteria (i.e., bacteria that transfer electrons using an extracellular mechanism). The fermenter consists of electrodes that act as sink and source for the production and movement of electrons and protons, thus generating electricity and producing valuable products. The conventional process of fermentation has several drawbacks that restrict their application and economic viability. Additionally, metabolic reactions taking place in traditional fermenters are often redox imbalanced. Almost all metabolic pathways and microbial strains have been studied, and EF can electrochemically control this. The process of EF can be used to optimize metabolic processes taking place in the fermenter by controlling the redox and pH imbalances and by stimulating carbon chain elongation or breakdown to improve the overall biomass yield and support the production of a specific product. This review briefly discusses microbe-electrode interactions, electro-fermenter designs, mixed-culture EF, and pure culture EF in industrial applications, electro methanogenesis, and the various products that could be hence generated using this process.

Anodic metabolic activity regulates the desalination efficiency in microbial catalysed electrochemical system

Anodic metabolic rate showed regulatory influence on the desalination performance of microbial desalination cell (MDC) under open (OC) and closed circuit (CC) operations. In this study, three MDCs were tested for desalination with three different organic substrate loads 1500 ± 55 mg/L in MDC-A; 3500 ± 10 g/L in MDC-B; 4500 ± 12 g/L in MDC-C. Higher desalination and substrate removal rates were observed in CC than OC. Average desalination was MDC-C (51.4%-CC) > MDC-B (47.3%-CC) > MDC-A (45.3%-CC) and COD removal efficiencies were MDC-C (68.4%-CC) > MDC-B (64.4%-CC) > MDC-A (51.9%-CC). Increase in organic load resulted in higher desalination efficiency which was due to higher electrochemical and ionic gradient apart from anodic metabolic activity. The voltage and current density were observed to be maximum in MDC-C (685 mV; 2.16 mA/m²) followed by MDC-B (598 mV; 1.98 mA/m²) and MDC-A (501 mV; 1.76 mA/m²). This study demonstrated that the MDCs performance can be regulated by varying organic load and circuitry modes.

Self-induced bioelectro-potential influence on sulfate removal and desalination in microbial fuel cell

This study focused on treatment of sulfate-rich produced water (PW) using microbial fuel cell (MFC) with biotic anode (bAC) and abiotic cathode (aCC) separated by a cation exchange membrane (CEM). MFC was operated under varied circuitry modes - open circuit (OC-without resistance) and closed circuit (CC-applied resistance (1kΩ)) to evaluate and assess the removal of sulfates and salts with simultaneous carbon utilization. The OC and CC operations depicted sulfates removal efficiency of 38% and 56%, salinity removal of 12% and 21% and COD removal of 47% and 58%, respectively. Both OC and CC showed K⁺ decrement in bAC and increment in aCC with a comparatively higher efficiency of ionic mobility in CC operation. Maximum open circuit voltage (OCV) of 498 mV (OC) was observed with redox catalytic peak currents from cyclic voltammetry [Anode/cathode, 3.5/-4.9 mA (OC); 6.9/-7.9 mA (CC)]. Dominance of Proteobacteria and Actinobacteria with specific enrichment of sulfate reducing bacteria (SRB) and halophiles was observed in bAC at the end of operation.

Mediated Fuel Cells: Soluble Redox Mediators and Their Applications to Electrochemical Reduction of O2 and Oxidation of H2, Alcohols, Biomass, and Complex Fuels

Mediated fuel cells are electrochemical devices that produce power in a manner similar to that of conventional proton exchange membrane fuel cells (PEMFCs). They differ from PEMFCs in their use of redox mediators dissolved in liquid electrolyte to conduct oxidation of the fuel or reduction of the oxidant, typically O2, in bulk solution. The mediators transport electrons (and often protons) between the electrode and the catalysts or chemical reagents in solution. This strategy can help overcome many of the challenges associated with conventional fuel cells, including managing complex multiphase reactions (as in O2 reduction) or the use of challenging or heterogeneous fuels, such as hydrocarbons, polyols, and biomass. Mediators are also commonly used in enzymatic fuel cells, where direct electron transfer from the electrode to the enzymatic active site can be slow. This review provides a comprehensive survey of historical and recent mediated fuel cell efforts, including applications using chemical and enzymatic catalysts.

Conductive magnetite nanoparticles trigger syntrophic methane production in single chamber microbial electrochemical systems

Performance of methane-producing microbial electrochemical systems (MESs) is highly reliant on electron transfer efficiency from electrode to microorganisms and vice versa. In this study, magnetite nanoparticles were used as electron carriers to enhance extracellular electron transfer in single chamber MESs. The MES with magnetite exhibited the highest methane yield and current generation of 0.37 ± 0.009 L_CH4/g_COD and 9.6 mA, respectively among the tested reactors. The experimental data was observed to be highly consistent with modified Gompertz model results (R² > 0.99), which also showed 74.2% and 22.1% enhanced methane production rate in MES with magnetite as compared to control AD and MES without magnetite, respectively. Cyclic voltammetry and electrochemical impedance spectroscopy analysis confirmed that magnetite enhanced catalytic activity of biofilm and lowered both solution and charge transfer resistance. Therefore, supplementing magnetite in MESs could be a strategy to develop an efficient syntrophic biomethanation in field scale applications.

A novel single chamber vertical baffle flow biocathode microbial electrochemical system with microbial separator

A 10-liter single chamber vertical baffle flow biocathode microbial electrochemical system (MES) with microbial separator was designed for wastewater treatment. The anode and cathode compartments were incompletely isolated by the microbial separator, which enabled module integration and centralized sludge collection of MES. The effluent COD was <50 mg L^-1 with COD removal of 86 ± 2% and low sludge yield rate of 0.05 ± 0.02 g-sludge g^-1 -COD. The MES performance was mainly restricted by biocathodes and supporting matrixes with higher permeability resulted in better cathode performance. The MES obtained the maximum power density of 67.5 ± 7.8 mW m^-2 with two layers of filter cloth and one layer of polyurethane sponge (S2P1) and supporting matrix with moderate permeability was more suitable in overall power generation and anode stability. The influences on bio-community of both cathodes and separators by the permeability of supporting matrixes were observed.

Remediation of Potential Toxic Elements from Wastes and Soils: Analysis and Energy Prospects

The aim of this study is to evaluate the application of the main hazardous waste management techniques in mining operations and in dumping sites being conscious of the inter-linkages and inter-compartment of the contaminated soils and sediments. For this purpose, a systematic review of the literature on the reduction or elimination of different potential toxic elements was carried out, focusing on As, Cd and Hg as main current contaminant agents. Selected techniques are feasible according to several European countries’ directives, especially in Spain. In the case of arsenic, we verified that there exists a main line that is based on the use of iron minerals and its derivatives. It is important to determine its speciation since As (III) is more toxic and mobile than As (V). For cadmium (II), we observed a certain predominance of the use of biotic techniques, compared to a variety of others. Finally, in mercury case, treatments include a phytoremediation technique using Limnocharis flava and the use of a new natural adsorbent: a modified nanobiocomposite hydrogel. The use of biological treatments is increasingly being studied because they are environmentally friendly, efficient and highly viable in both process and energy terms. The study of techniques for the removal of potential toxic elements should be performed with a focus on the simultaneous removal of several metals, since in nature they do not appear in isolation. Moreover, we found that energy analysis constitutes a limiting factor in relation to the feasibility of these techniques.

Purification and Characterization of NDH-2 Protein and Elucidating Its Role in Extracellular Electron Transport and Bioelectrogenic Activity

In microbial electrochemical systems, transport of electrons from bacteria to an electrode is the key to its functioning. However, the roles of several electron transport proteins, especially the membrane-bound dehydrogenases which link cellular metabolism to EET pathway are yet to be identified. NDH-2 is a non-proton pumping NADH dehydrogenase located in the inner membrane of several bacteria like Bacillus subtilis, Escherichia coli, etc. Unlike NADH dehydrogenase I, NDH-2 is not impeded by a high proton motive force thus helping in the increase of metabolic flux and carbon utilization. In the current study, NADH dehydrogenase II protein (NDH-2) was heterologously expressed from B. subtilis into E. coli BL21 (DE3) for enhancing electron flux through EET pathway and to understand its role in bioelectrogenesis. We found that E. coli expressing NDH-2 has increased the electron flux through EET and has shown a ninefold increase in current (4.7 μA) production when compared to wild strain with empty vector (0.52 μA). Furthermore, expression of NDH-2 also resulted in increased biofilm formation which can be corroborated with the decrease in charge transfer resistance of NDH-2 strain and increased NADH oxidation. It was also found that NDH-2 strain can reduce ferric citrate at a higher rate than wild type strain suggesting increased electron flux through electron transport chain due to NADH dehydrogenase II activity. Purified NDH-2 was found to be ∼42 kDa and has FAD as a cofactor. This work demonstrates that the primary dehydrogenases like NADH dehydrogenases can be overexpressed to increase the electron flux in EET pathway which can further enhance the microbial fuel cells performance.

Thermophiles; or, the Modern Prometheus: The Importance of Extreme Microorganisms for Understanding and Applying Extracellular Electron Transfer

Approximately four billion years ago, the first microorganisms to thrive on earth were anaerobic chemoautotrophic thermophiles, a specific group of extremophiles that survive and operate at temperatures ∼50 - 125°C and do not use molecular oxygen (O2) for respiration. Instead, these microorganisms performed respiration via dissimilatory metal reduction by transferring their electrons extracellularly to insoluble electron acceptors. Genetic evidence suggests that Gram-positive thermophilic bacteria capable of extracellular electron transfer (EET) are positioned close to the root of the Bacteria kingdom on the tree of life. On the contrary, EET in Gram-negative mesophilic bacteria is a relatively new phenomenon that is evolutionarily distinct from Gram-positive bacteria. This suggests that EET evolved separately in Gram-positive thermophiles and Gram-negative mesophiles, and that EET in these bacterial types is a result of a convergent evolutionary process leading to homoplasy. Thus, the study of dissimilatory metal reducing thermophiles provides a glimpse into some of Earth's earliest forms of respiration. This will provide new insights for understanding biogeochemistry and the development of early Earth in addition to providing unique avenues for exploration and discovery in astrobiology. Lastly, the physiological composition of Gram-positive thermophiles, coupled with the kinetic and thermodynamic consequences of surviving at elevated temperatures, makes them ideal candidates for developing new mathematical models and designing innovative next-generation biotechnologies. KEY CONCEPTS Anaerobe: organism that does not require oxygen for growth. Chemoautotroph: organism that obtains energy by oxidizing inorganic electron donors. Convergent Evolution: process in which organisms which are not closely related independently evolve similar traits due to adapting to similar ecological niches and/or environments. Dissimilatory Metal Reduction: reduction of a metal or metalloid that uses electrons from oxidized organic or inorganic electron donors. Exoelectrogen: microorganism that performs dissimilatory metal reduction via extracellular electron transfer. Extremophiles: organisms that thrive in physical or geochemical conditions that are considered detrimental to most life on Earth. Homoplasy: a character shared by a set of species that is not shared by a common ancestor Non-synonymous Substitutions (K a ): a substitution of a nucleotide that changes a codon sequence resulting in a change in the amino acid sequence of a protein. Synonymous Substitutions (K s ): a substitution of a nucleotide that may change a codon sequence, but results in no change in the amino acid sequence of a protein. Thermophiles: a specific group of extremophiles that survive and operate at temperatures ∼50-125°C.

Effects of the presence of nanoscale zero-valent iron on the degradation of polychlorinated biphenyls and total organic carbon by sediment microbial fuel cell

The degradation of polychlorinated biphenyls (PCBs) and total organic carbon (TOC) by sediment microbial fuel cell (SMFC) with/without nanoscale zero-valent iron (NZVI) addition was investigated. It was found that the combined application led to the highest removal efficiencies of PCBs (37.55 ± 1.11%) and TOC (49.72 ± 1.54%) in all circumstances and produced a higher power density (108.89 mW/m²) and a corresponding lower internal resistance (264 Ω) than operation employing SMFC only. The TOC removal efficiency and the total production of electricity were linear. High-throughput sequencing of anodic microbial communities indicated that the electrode participation can increase the abundance of electrogenic bacteria (Geobacter and Pseudomonas) and the NZVI addition can reduce the oxidation reduction potential of the system and therefore enrich some bacteria (Longilinea and Desulfofustis) beneficial to the degradation of organic matter.

Approaches and processes for ammonia removal from side-streams of municipal effluent treatment plants

The main objective of this review article is to provide a comprehensive view on various conventional and emerging side-stream ammonia removal treatment options for municipal wastewater treatment plants (WWTPs). Optimization of wastewater treatment facilities from an energy and emissions stand-point necessitates consideration of the impact of the various internal side-streams. Side-streams from anaerobic sludge digesters in particular have the potential to be a significant ammonium load to the mainstream treatment process. However, the literature suggests that managing side-streams through their treatment in the mainstream process is not the most energy efficient approach, nor does it allow for practical recovery of nutrients. Furthermore, as effluent criteria become more stringent in some jurisdictions and sludge hydrolysis pre-treatment for digesters more common, an understanding of treatment options for ammonia in digester supernatant becomes more important. Given these considerations, a variety of side-stream treatment processes described in the literature are reviewed.

Electrochemical Bioreactor Technology for Biocatalysis and Microbial Electrosynthesis

Two seemingly distinct fields, industrial biocatalysis and microbial electrosynthesis, can be viewed together through the lens of electrochemical bioreactor technology in order to highlight the challenges that exist in creating a versatile platform technology for use in chemical and biological applications. Industrial biocatalysis applications requiring NAD(P)H to perform redox transformations often necessitate convoluted coupled-enzyme regeneration systems to regenerate reduced cofactor, NAD(P)H from oxidized cofactor, NAD(P). Renewed interest in continuously recycling the cofactor via electrochemical reduction is motivated by the low cost of performing electrochemical reactions, easy monitoring of the reaction progress, and straightforward product recovery. However, electrochemical cofactor regeneration methods invariably produce adventitious reduced cofactor side products which result in unproductive loss of input NAD(P). Microbial electrosynthesis is a form of microbially driven catalysis in which electricity is supplied to living microorganisms for the production of industrially relevant chemical products at higher carbon efficiencies and yields compared with traditional, nonelectrically driven, fermentations. The fundamental biochemistry of these organisms as related to selected biochemical redox processes will be explored in order to highlight opportunities to devise strategies for taking advantage of these biochemical processes in engineered systems.

Degradation of 2,4,6-trichlorophenol and determination of bacterial community structure by micro-electrical stimulation with or without external organic carbon source

This study aimed to investigate the degradation efficiency of 2,4,6-trichlorophenol through a batch of potentiostatic experiments (0.2 V vs. Ag/AgCl). Efficiencies in the presence and absence of acetate and glucose were compared through open-circuit reference experiments. Significant differences in degradation efficiency were observed in six reactors. The highest and lowest degradation efficiencies were observed in the closed-circuit reactor fed with glucose and in the open-circuit reactor, respectively. This finding was due to the enhanced bacterial metabolism caused by the application of micro-electrical field and degradable organics as co-substrates. The different treatment efficiencies were also caused by the distinct bacterial communities. The composition of bacterial community was affected by adding different organics as co-substrates. At the phylum level, the most dominant bacteria in the reactor with the added acetate and glucose were Proteobacteria and Firmicutes, respectively.

Stacking of microbial fuel cells with continuous mode operation for higher bioelectrogenic activity

The effect of stacking multiple microbial fuel cells for stable power output was evaluated in continuous mode operation. Three single chambered air cathode CMFCs with Nafion (CMFC_N), Terry cotton (CMFC_T) as membranes and one without membrane (CMFC_ML) were operated in continuous mode. Maximum power density (PD) and COD removal efficiency was obtained for CMFC_N (0.1 W/m², 50%) followed by CMFC_ML (0.062 W/m², 47%) and CMFC_T (0.025 W/m², 39%) and were stable throughout the operation. To increase the power output further, stacking of CMFCs was carried in series/parallel circuitry, which yielded high power density in parallel (2.0 W/m²; 7.2 W/m³) and high voltage in series (1.1 V). Study also evidenced that stacking resulted in high and stable bioelectricity by minimizing the electron losses in comparison to individual CMFCs operation. Stable and high power output signifies the impact of continuous mode operation that constantlty replenishes the substrate.

Rewiring the microbe-electrode interfaces with biologically reduced graphene oxide for improved bioelectrocatalysis

The aim of this work was to study biologically reduced graphene oxide (RGO) for engineering the surface architecture of the bioelectrodes to improve the performance of Bioelectrochemical System (BES). Gluconobacter roseus mediates the reduction of graphene oxide (GO). The RGO modified bioelectrodes produced a current density of 1 mA/cm² and 0.69 mA/cm² with ethanol and glucose as substrates, respectively. The current density of RGO modified electrodes was nearly 10-times higher than the controls. This study, for the first time, reports a new strategy to improve the yield as well as efficiency of the BES by wrapping and wiring the electroactive microorganisms to the electrode surfaces using RGO. This innovative wrapping approach will decrease the loss of electrons in the microbe-electrolyte interfaces as well as increase the electron transfer rates at the microorganism-electrode interfaces.

Spatial variation of electrode position in bioelectrochemical treatment system: Design consideration for azo dye remediation

In the present study, three bio-electrochemical treatment systems (BET) were designed with variations in cathode electrode placement [air exposed (BET1), partially submerged (BET2) and fully submerged (BET3)] to evaluate azo-dye based wastewater treatment at three dye loading concentrations (50, 250 and 500 mg L^-1). Highest dye decolorization (94.5 ± 0.4%) and COD removal (62.2 ± 0.8%) efficiencies were observed in BET3 (fully submerged electrodes) followed by BET1 and BET2, while bioelectrogenic activity was highest in BET1 followed by BET2 and BET3. It was observed that competition among electron acceptors (electrode, dye molecules and intermediates) critically regulated the fate of bio-electrogenesis to be higher in BET1 and dye removal higher in BET3. Maximum half-cell potentials in BET3 depict higher electron acceptance by electrodes utilized for dye degradation. Study infers that spatial positioning of electrodes in BET3 is more suitable towards dye remediation, which can be considered for scaling-up/designing a treatment plant for large-scale industrial applications.

Microbial electrohydrogenesis linked to dark fermentation as integrated application for enhanced biohydrogen production: A review on process characteristics, experiences and lessons

Microbial electrohydrogenesis cells (MECs) are devices that have attracted significant attention from the scientific community to generate hydrogen gas electrochemically with the aid of exoelectrogen microorganisms. It has been demonstrated that MECs are capable to deal with the residual organic materials present in effluents generated along with dark fermentative hydrogen bioproduction (DF). Consequently, MECs stand as attractive post-treatment units to enhance the global H₂ yield as a part of a two-stage, integrated application (DF-MEC). In this review article, it is aimed (i) to assess results communicated in the relevant literature on cascade DF-MEC systems, (ii) describe the characteristics of each steps involved and (iii) discuss the experiences as well as the lessons in order to facilitate knowledge transfer and help the interested readers with the construction of more efficient coupled set-ups, leading eventually to the improvement of overall biohydrogen evolution performances.

Food waste biorefinery: Sustainable strategy for circular bioeconomy

Enormous quantity of food waste (FW) is becoming a global concern. To address this persistent problem, sustainable interventions with green technologies are essential. FW can be used as potential feedstock in biological processes for the generation of various biobased products along with its remediation. Enabling bioprocesses like acidogenesis, fermentation, methanogenesis, solventogenesis, photosynthesis, oleaginous process, bio-electrogenesis, etc., that yields various products like biofuels, platform chemicals, bioelectricity, biomaterial, biofertilizers, animal feed, etc can be utilized for FW valorisation. Integrating these bioprocesses further enhances the process efficiency and resource recovery sustainably. Adapting biorefinery strategy with integrated approach can lead to the development of circular bioeconomy. The present review highlights the various enabling bioprocesses that can be employed for the generation of energy and various commodity chemicals in an integrated approach addressing sustainability. The waste biorefinery approach for FW needs optimization of the cascade of the individual bioprocesses for the transformation of linear economy to circular bioeconomy.

Carbon dioxide capture, storage and production of biofuel and biomaterials by bacteria: A review

Due to industrialization and urbanization, as humans continue to rely on fossil fuels, carbon dioxide (CO₂) will inevitably be generated and result in an increase of Global Warming Gases (GWGs). However, their prospect is misted up because of the environmental and economic intimidation posed by probable climate shift, generally called it as the "green house effect". Among all GWGs, the major contributor in greenhouse effect is CO₂. Mitigation strategies that include capture and storage of CO₂ by biological means may reduce the impact of CO₂ emissions on environment. The biological CO₂ sequestration has significant advantage, since increasing atmospheric CO₂ level supports productivity and overall storage capacity of the natural system. This paper reviews CO₂ sequestration mechanism in bacteria and their pathways for production of value added products such as, biodiesel, bioplastics, extracellular polymeric substance (EPS), biosurfactants and other related biomaterials.

Toward establishing minimum requirements for extracellular electron transfer in Geobacter sulfurreducens

The highly redundant pathways for extracellular electron transfer in Geobacter sulfurreducens must be simplified for this microorganism to serve as an effective chassis for applications such as the development of sensors and biocomputing. Five homologs of the periplasmic c-type cytochromes, PpcA-E, offer the possibility of multiple routes of electron transfer across the periplasm. The presence of a large number of outer membrane c-type cytochromes allows G. sulfurreducens to adapt to disruption of an electron transfer pathway in the outer membrane. A strain in which genes for all five periplasmic cytochromes, PpcA-E, were deleted did not reduce Fe(III). Introducing ppcA under the control of an IPTG-inducible system in the quintuple deletion strain yielded a strain that reduced Fe(III) only in the presence of IPTG. A strain lacking known major outer membrane cytochromes, OmcB, OmcE, OmcS and OmcT, and putative functional homologs of OmcB, did not reduce Fe(III). Introduction of omcB in this septuple deletion strain restored the ability to reduce Fe(III). These results demonstrate that it is possible to trim redundancy from the extracellular electron transfer pathways in G. sulfurreducens in order to construct strains with defined extracellular electron transfer routes.